Handy solar info for lead acid batteries

[hbski]

Handy Bob

[landyacht318]

I read HB's rant a long while back. What I took away from it was to use fatter cable/wire and have the Controller as close to the batteries as possible and bump up absorption voltage.

Makes perfect sense and I have no problem with perfect sense.

What I do have a problem with is when some people state their opinions as facts, and that anybody who disagrees is a knuckle dragging drooler.

Often this seems to stem from an incredible insecurity, and getting Kudos from the masses of faithful internet sheep, bolsters their self esteem, and their perhaps misguided faith in their own beliefs and inflated sense of self importance.

Such people I want to tell to shut the front door, go take another selfie, post it online and count your thumbs up replies as that is apparently the societal zeitgeist of current culture.

Just don't do it sitting at a traffic light and get indignant when a horn reminds you that you have a responsibility to actually drive and pay attention to something other than you own selfish immediate interests.

[Salvo]

A criticism of pwm performance is that temperature can drop PV voltage below battery voltage. This criticism is make when a 36 cell panel charges a 12V battery, or a 72 cell panel charges a 24V battery.

BLF has a 24V panel that has only 60 cells, not 72 cells. That means his panel voltage is 1 - 60/72 = 17% lower than a 72 cell panel. That panel will more likely run out of voltage headroom in hot weather than the 72 cell panel.

Victron white paper

BFL13 wrote:

Salvo wrote:
Your 24V panels are not "true" 24V panels. A true 24V panel has 2 * 36 cells. You have less. That means the cliff where current drops is more to the left, at a lower voltage. Add the effect of temperature and your 24V pwm controller will have problems at higher temperature.

I was able to get the 24v battery bank to 30 volts before it ran out of steam due to the overhead, so it is not true that a 60 cell panel can't fully charge a 24v battery.

[pianotuna]

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[Salvo]

Exactly.

Stating HB is dead wrong does a disservice. He deserves respect.

hbski wrote:


When your small percentages apply to the OVERALL voltages, then yes they are small. HOWEVER, when you apply these differences to the VOLTAGE THAT MATTERS (ie the voltage difference between charging voltage and battery voltage the difference that will actually push AMPS into the batteries) then these "small" losses become MUCH more important.

[BFL13]

Salvo wrote:
Your 24V panels are not "true" 24V panels. A true 24V panel has 2 * 36 cells. You have less. That means the cliff where current drops is more to the left, at a lower voltage. Add the effect of temperature and your 24V pwm controller will have problems at higher temperature.

BFL13 wrote:

Since PWM uses Isc, which does not go down with temperature, you don't care about panel temp with PWM, but it hurts MPPT big time.

The conversation is more about wire gauge but as noted you also get voltage loss from panel heating. There is another issue with the "overhead" reduction using 60 cell 24v panels instead of 72 cell panels so any voltage loss is more critical with 60 cell panels.

I am on limited data so somebody could add my thread on this here as a link. "Hot Solar Panel MPPT vs PWM Results" from last June.

A mystery I never solved was the Isc with 24v as noted in that OP, where Isc was less than rated using 24-24, but as rated using 24-12.

Panel Isc is rated as 9.0a and I was getting just above that, showing that insolation was good at the time of the test.

Disconnected panel Isc was 9.16 to 9.22 and the MPPT controller's "demo" showed that 9.16 as panel amps when doing 24-12.

However, doing 24-24 the demo amps were only 8.02. Meanwhile the PWM controller doing 24-24 also showed only 8.1 amps.

This was a disappointment since I expected the PWM to get Isc as it does with 12-12, so I was expecting to get 9 amps in 24-24( which would be like 18 amps in 12-12) and that would beat the MPPT doing 16 amps. Nope. Both got "16" in 24-24

I thought it would be due to the 60 cell panel vs 72 cell, but the MPPT controller did show Isc at the full 9 amps when in 24-12 mode.

Both controllers showed 8 amps Isc as an upper limit in 24-24. I don't understand how the lower "overhead" of the 60 cell panel would manifest as a lower Isc.

I was able to get the 24v battery bank to 30 volts before it ran out of steam due to the overhead, so it is not true that a 60 cell panel can't fully charge a 24v battery. No way to get over that 30 and equalize though. (it seems 60 cell is the usual way 24v panels come these days. (If you want 72, and get two 12s in series, beware of the total Voc wrt your controller's limit on that)

[red31]

The alternative link suggests #4 for 12v

http://www.jackdanmayer.com/rv_electrical_and_solar.htm#Why_Many_Solar_Systems_Do_Not_Work_Well

They both actually suggest using voltage drop calculators or tables and 14.8v set point.

Worse case may be when equalize is needed on a hot day, ain't that why I went w/4A per 100 ah.

[westend]

One result of Bob's articles may be to sway a solar user to use larger wire. That, of itself, is a good result. I can't understand why a user will spend hundreds on modules and a controller but won't spend another $20-$50 on wire to insure the most harvest possible.

[hbski]

I'm the OP and was only trying to provide some handy info, if for no other reason than for folks to gain perspective. Perspective is AWESOME because even if you don't agree with the other point of view, it still gives you very valuable information....data points!!!

Many responders have indicated that HB's info is dated. To which I will wholeheartedly agree. That said, how many RV'rs are on the bleeding edge (or even REMOTELY so) with their solar???? Pretty sure his "dated" info applies to 90% of RV folks.

[hbski]

pianotuna wrote:
"All solar panels lose power at higher cell temperatures. This is why most panels are designed for 16.5 to 17.5 volts output at room temperature - when the panel cell temperatures get up to 150 degrees F or so, voltage output can drop as much as 20%"

150 f = 65 C.

If the panel is putting out 17.5 volts @ 25 C then at 65 C the voltage would still be 14.

Voltage drop at 14 for a 180 watt panel which would service 300 amp-hours of battery bank with a 15 foot run would amount to a further 0.31 volts drop with #8 wire. That still leaves the charging voltage at a comfortable 13.69 volts input to the controller.

Therefore for a 180 watt system #8 wire would work well.

Going down in size to #10 wire "costs" an additional .25 volts (.56) leaving 13.44 volts to the controller.

Going down to #12 "costs" a total of .84 leaving 13.16 to the controller. Still adequate but not "great".

Going to #14 costs 1.4 volts leaving 12.6 volts to the controller. That's not good.

This is pretty much based on a 'worst case' scenario where the panels hit 65 C (150 f). I think that is unlikely, at least where I live.

Another solution would be to use a higher voltage panel.

If I remember correctly handy suggests wire sizes much larger than those I've listed.

In any event, if you want low line losses go MPPT and get the voltage up to say 48, or even 140. You will pay for a more expensive controller to do so--and probably it is not worth it until about a 600 watt threshold.

When your small percentages apply to the OVERALL voltages, then yes they are small. HOWEVER, when you apply these differences to the VOLTAGE THAT MATTERS (ie the voltage difference between charging voltage and battery voltage the difference that will actually push AMPS into the batteries) then these "small" losses become MUCH more important.

[time2roll]

I think will stick with 24 volt panels.

[pianotuna]

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[Salvo]

Your 24V panels are not "true" 24V panels. A true 24V panel has 2 * 36 cells. You have less. That means the cliff where current drops is more to the left, at a lower voltage. Add the effect of temperature and your 24V pwm controller will have problems at higher temperature.

BFL13 wrote:

Since PWM uses Isc, which does not go down with temperature, you don't care about panel temp with PWM, but it hurts MPPT big time.

[Salvo]

No, I'm not talking about 10% reduction in power. I'm talking about close to 100% reduction.



We've been over this already.

This chart doesn't have a voltage scale. Let's say the point where the two curves cross is at 14.5V. The panel outputs 10A in pwm mode. The cable voltage drop is 0.3V. When the battery gets up to 14.2V, current starts to back off from 10A and falls off the cliff. The bigger the cable, the greater the battery voltage gets before current falls off the cliff.

That's just an example with made up numbers. Get the PV voltage temperature coefficient spec for real quantities.

[BFL13]

PT, I don't know what you and Salvo are on about, but for instance, my 255w panel mounted out on the grass away from the trailer on a tilted up contraption with lots of breeze off the ocean, was 50C with 25c ambient, where I measured using an IR gun gizmo pointing up underneath a foot away from the white backing. At 30+C I saw 55C panel underneath.

BTW the panel temp varies where you aim the gun. Sort of varies side to side and up to down. I couldn't solve that by swapping the panel end to end either. Might be an E-W thing, don't know. Anyway, that means each cell is at a different temp so its voltage will be different and the panel voltage is the result of all that added up.

Since PWM uses Isc, which does not go down with temperature, you don't care about panel temp with PWM, but it hurts MPPT big time. There you use input watts and output watts divided by batt V to get charging amps. That means a higher panel temp lowers panel V and so input watts and so charging amps in turn.